The term "family TV" runs through the history of the medium as both a promise from executives – the coveted franchises with a nine-to-90 age-range such as Strictly Come Dancing – and a journalistic curse: when contestants thrust or touch suggestively in shows such as The X Factor and Britain's Got Talent, the makers are accused of breaking the watershed or the rules of communal viewing.

But there is also a genre of programming that has the family as its subject matter and three new examples start this week, although, ironically, two of them start after the 9pm watershed that designates the end of the shows that everyone can watch.

Visibly influenced by the slew of American documentaries about pushy parents coaching their offspring through spelling bees or fashion pageants, the four-part documentary follows some of the contestants in a competition run by Mensa to find Britain's brightest child, through a series of maths and logic tests.

Though technically an observational documentary, the series feels like a freak show, seemingly cast to select the most revoltingly precocious children – reflecting on the difficulties of interacting with "stupid" peers – and the most alarmingly ambitious parents, including a mum who, when her son vomits on the morning of the competition, tells him that top athletes often throw up before a race.

Daniel Radcliffe recently joked that the eight Harry Potter films serve as a sort of blockbuster home-movie set, charting his childhood, but that he is too embarrassed to look at them. You fear that, when the subjects of Child Genius are adults, they will regard this series as a sort of home-video nasty. The same is probably true of some of those whose folks forced them to appear on the old showoff show Ask the Family but, these days, it's even worse because the most gruesome clips of tantrums and anti-cuteness will be preserved for ever online, ready to be consulted by prospective dates and employers.

Watson's contract with his subjects included the clause that they would not be filmed while using the lavatory or having sex. After four decades of broadcasting that have included Geordie Shore and The Only Way is Essex, producers these days would probably need forcibly to persuade some reality TV participants not to copulate or defecate on camera.

Whereas Watson's show caught the formlessness of everyday life – there were editions in which nothing much happened except some shopping or a conversation – Happy Families clearly shows the influence of constructed reality shows such as TOWIE and Made in Chelsea by inter-twining the footage after the event into narratives in which something – a cash-flow problem, a medical scare – is always happening.

However, despite the crass jollity of the voiceover ("Families – we've all got one!") the scenes frequently feel authentic and, in one case, almost unbearably tragic to watch. During that particular sequence, many viewers may worry about whether we should really be seeing this. Commentators have argued that the military interventions in Kosovo, Afghanistan and Iraq resulted from a "Schindler's List generation" of politicians, who were driven to early intervention against potential genocide. Watching Happy Families and Child Genius, you suspect the existence of a "Truman Show generation" among the TV audience, who are happy for almost anything to be seen on screen.

The only one of these programmes about families to be shown in the family viewing zone is – paradoxically – the show that under-16s would be least interested in seeing. Compare Your Life (C4, Thursday, 9pm) is, from the evidence of an incomplete sample episode, a variation on the Location, Location, Location genre of lifestyle aspiration shows.

Given that home-buying shows have been trimmed from the schedules because of the property and mortgage crisis, it seems slightly odd that the alternative should be a series in which couples are offered a choice of three different new family lifestyles, which involve a home move and taking over a business. The gimmick is that presenter Carlton Hood, founder of a comparison website, uses "scientific" methods including a "priority analysis test" to help them to choose.

TV critics have nothing as fancy as that. But, based on the preview material available, if you want to watch a factual family show this week, I would rank this three in order: 1) Happy Families, 2) Child Genius, 3) Compare Your Life.

David Tennant is appearing in The Politician's Husband tonight, days after wrapping up Broadchurch with Olivia Colman. But there's no danger of overexposure for either of these brilliantly versatile performers ...